The term additive manufacturing describes technologies which can be used anywhere throughout the product life cycle from pre-production (i.e. rapid prototyping) to full scale production (also known as rapid manufacturing) and even for tooling applications or post production customization. Accordingly, Additive manufacturing (AM) is defined as the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining. The first successful attempts at additive manufacturing came from technology developed in the 1970s, and since then the additive manufacturing technology is developing rapidly with increasing industrial applications. Nowadays, there are already a variety of AM techniques being developed, such as three-dimensional printing (3DP), stereo lithography apparatus (SLA), laminated object manufacturing (LOM), selective laser sintering (SLS), selective laser melting (SLM), and fused deposition modeling (FDM), etc.
Currently, the most well-known additive manufacturing technique is the laser-based additive manufacturing (LBAM) technique, such as the selective laser sintering (SLS) and the selective laser melting (SLM). Taking a SLM process for example, it is performed using a high power laser to fuse small particles of plastic, metal, ceramic, or glass powders into a mass that has a desired 3-dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed. It is noted that LBAM can produce relatively complex three-dimensional structures like inner cavities and inner channels, which are normally difficult or impossible for traditional manufacturing technologies based on material removal. However, there are still issues to be solved for improving the current additive manufacturing technique, such as how to increase the density of the part being built, how to increase the uniformity of deposition, how to get improvement in the geometrical accuracy of the part being built, and how to eliminate the thermal deformation due to the temperature variation in the ambience, and so on.
There are already many studies focused upon the improvement of additive manufacturing technique. One of which is an apparatus for producing parts by selective sintering, disclosed in U.S. Pat. No. 5,594,589, in which a powder dispensing mechanism including a drum is used and enabled to move horizontally across the target area and counter-rotated to smooth and distribute the powder in an even layer across the target area, while also a downdraft system is used for providing a controlled temperature air flow through the target area to moderate powder temperature during sintering. Moreover, there is a coating device provided in U.S. Pat. No. 5,730,925, which is configured with a bevel scraper or a round scraper to be used for applying, smoothing and compacting the solidified material on a layer. Similarly, there is an applicator unit disclosed in U.S. Pat. No. 7,047,098, in which a flexible means is used for applying a powder layer to a layer below it. In addition, in U.S. Pat. No. 7,048,530, a shoe with a working surface having specific indentations formed thereon to be used for applying a powder layer is disclosed, which is arranged and adapted to rotate and incline at an angle so as to spread and thus apply the powder layer.